The present invention relates generally to growth of semiconductor crystals. More particularly, the present invention relates to a procedure for in-situ determination of thermal gradients at the crystal growth front in a semiconductor crystal.
Most processes for fabricating semiconductor electronic components are based on single crystal silicon. Conventionally, the Czochralski process is implemented by a crystal pulling machine to produce an ingot of single crystal silicon. The Czochralski or CZ process involves melting highly pure silicon or polycrystalline silicon in a crucible located in a specifically designed furnace contained in part by a heat shield. The crucible is typically made of quartz or other suitable material. After the silicon in the crucible is melted, a crystal lifting mechanism lowers a seed crystal into contact with the silicon melt. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt. The crystal is substantially free of defects and therefore suitable for manufacturing modern semiconductor devices such as integrated circuits. While silicon is the exemplary material in this discussion, other semiconductors such as gallium arsenide, indium phosphide, etc. may be processed in similar manner, making allowances for particular features of each material.
A key manufacturing parameter is the diameter of the ingot pulled from the melt. After formation of a crystal neck or narrow-diameter portion, the conventional CZ process enlarges the diameter of the growing crystal. This is done under automatic process control by decreasing the pulling rate or the temperature of the melt in order to maintain a desired diameter. The position of the crucible is adjusted to keep the melt level constant relative to the crystal. By controlling the pull rate, the melt temperature, and the decreasing melt level, the main body of the crystal ingot grows with an approximately constant diameter. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable or shaft along with the seed and the crystal in an opposite direction.
Conventionally, the Czochralski process is controlled in part as a function of the diameter of the crystal during pulling and the level of molten silicon in the crucible. Process goals are a substantially uniform crystal diameter and minimized crystal defects. Crystal diameter has been controlled by controlling the melt temperature and the pull speed.
It has been found that temperature gradient at the crystal growth front (i.e., the crystal-melt interface) is also a valuable measure of process performance. Temperature gradients are important crystal growing process parameters that affect crystal diameter control, crystal morphological stability in heavily doped crystal growing, and bulk crystal micro-defects. Conventionally, nominal temperature gradients are pre-determined by hot-zone design, which is done with the help of computer assisted design (CAD) software. Later in praxis, the actual gradients, without really knowing precise values, are then adjusted (e.g. by making small changes to the melt-heat-shield-gap) according to post pull material analysis, for instance by analyzing the distribution of interstitial and vacancy defects. Such adjustments are done on a run-by run basis and a number of high quality CZ-materials with tight material properties specifications require permanent monitoring and adjustment. The permanent monitoring is necessary because material properties of the hot-zone parts that determine the thermal gradients change over time due to repeated use. However, such run-to-run analysis is unable to fine-adjust gradual changes that occur during a run and, worse, it is unable to catch and correct during a run gradient deviations due to pre-run set-up-errors such as a wrong melt-heat-reflector gap, etc., that sometimes occur due to human error. What is needed is a reliable method and apparatus for determining thermal gradients at the crystal growth front during crystal growth and for controlling the crystal growth process using this information.